SQL expressions: allow usage with server-side expressions

While trying to use SQL expressions to strip labels from the results of alerting queries, I ran into the following error: > “only data source queries may be inputs to a sql expression.” This happened when applying a SQL expression to the output of a Reduce expression. <img width="781" height="499" alt="Image" src="https://github.com/user-attachments/assets/9a3a27b6-3468-490f-9ed5-759059a62c55" /> This was initially reported in https://github.com/grafana/grafana/issues/103291 and is related to https://github.com/grafana/grafana/issues/104054. cc @samjewell